Investigation of ion-ion interaction effects on Yb³⁺-doped fiber amplifiers

Ytterbium (Yb³⁺)-doped materials have been widely used for high efficiency high energy laser sources at the 1 µm wavelength region because of their very low quantum defect and the unique simple energy level structure of Yb³⁺, resulting in no excited-state absorption and low occurrence probability of deleterious ion-ion interaction processes. It has been generally recognized that these ion-ion interaction processes have very little influence on the operation of Yb³⁺-doped fiber lasers at low and moderate power levels. However, our recent study shows that the performance of Yb³⁺-doped fiber amplifiers operating at low power levels is still influenced by the ion-ion interaction processes due to the large amount of population at the upper laser level ²F_5/2. In this paper, experimental evidences of the ion-ion interaction effects in Yb³⁺-doped fiber amplifiers are presented and a new model including these effects is developed for the numerical simulation. Our experimental and numerical investigations on the 976 nm and 1030 nm Yb³⁺-doped silica and phosphate fiber amplifiers show that ion-ion interaction has non-negligible impact on the performance of Yb³⁺-doped fiber amplifiers indeed, and compared to Yb³⁺-doped silica fibers, Yb³⁺-doped phosphate fibers suffer much less from the ion-ion interaction effects due to the much less clustered ions.